My invention relates to internal-combustion engines of the type employing a motor piston that reciprocates in a cylinder while turning a crankshaft for the extraction of motive power from the expansion of hot gases produced by the combustion of a compressed fuel-air charge that is supplied under pressure to a combustion chamber by a charger piston. A type fitting into the broad classification of two-stroke pump-compression cycle internal-combustion engines.
Currently, most heat engines in use for mechanical power generation are of the four-stroke cycle type that was patented by Alphonse Beau de Rochas in 1862 and adapted for manufacture and practical use in 1876 by Nicolaus August Otto. These engines have been very successful for over a century and they are still the standard of comparison. Their mode of operation appears quite logical; but they do not make maximum use of the heat that is generated by the combustion of a fuel-air charge. And the various two-stroke cycle gasoline engines in current use suffer from the same problem. In the four-stroke Otto cycle, a fuel-air mixture is drawn into a cylinder having a combustion chamber into which the mixture is compressed by a piston. The fuel-air mixture is then ignited, and the heat released from burning the fuel greatly increases the pressure of the resulting gases by raising them to a very high temperature. The extremely hot high-pressure gases then push the piston down to turn the crankshaft and do work. But paradoxically, in Otto cycle engines all parts of the combustion chamber must be kept reasonably cool. If they are not cooled, heat absorbed from the burning gases during the power strokes will increase the temperature of the parts until something becomes so hot that it will ignite the fuel-air charge during the induction stroke. Ignition then results in a backfire explosion through the intake manifold that prevents the successful completion of the cycle. But even before any part becomes that hot, there will be preignition during the compression stroke that is likely to cause power loss, detonation and overheating of the piston and valves that can destroy the engine. Since it is essential that the combustion chamber of these engines be kept cool, it is necessary to incorporate an air or liquid cooling system that usually adds substantially to their bulk and complexity and detracts from their reliability. However, the combustion and expansion phase of the cycle would be far more efficient if it were to be conducted using a very hot insulated chamber. Then the extremely hot high-pressure combustion gases would not as quickly transfer the heat, which produced and maintains the high pressure, to the internal parts of the combustion chamber. These include the cylinder head, the valves, the piston crown, the spark plug and the exposed part of the cylinder wall.
An answer to the problem might be to induct only air and then inject the fuel into it after it is compressed, as in a Diesel engine. This will solve the preignition problem; however, if the parts in the combustion chamber are allowed to become very hot, the air will be heated and expanded as it enters the cylinder, so less air will enter. This will reduce the mass of air available for combustion. Also, if the air is being heated by the chamber surfaces while being compressed, more work must be expended to compress it. The efficiency then is likely to get worse instead of better as was shown by the research in the Adiabatic Engine Program.
Since a cool chamber is needed for induction, but a hot chamber is needed for expansion, a new approach is necessary before the thermal efficiency can be raised to a much higher level. The dilemma can be resolved by making an engine that inducts and compresses its fresh charge in a separate cavity. Then, at the proper time, the fresh charge is forced into a combustion chamber, which is insulated and kept very hot, for combustion and expansion. This will slow the rapid loss of heat from the combustion gases so that the pressure does not drop as fast while the piston is being pushed down. If less heat is lost, a higher expansion ratio can then be used to extract more mechanical energy from the heat energy.
The only way that it is possible to increase the indicated thermal efficiency of internal-combustion piston engines is to reduce the amount of heat wasted. This is true regardless of whether the waste is from incomplete combustion, forced convection followed by conduction through the chamber surfaces or out the exhaust pipe following a low ratio expansion. Increasing the expansion ratio will be of limited benefit in four-stroke spark-ignition engines where purging of heat from the combustion chamber is practiced because it is intrinsically necessary. And it is unlikely that the mechanical efficiency will ever be much improved while a four-stroke engine requiring two revolutions per cycle, an extensive valve train and a mechanically pumped and fanned liquid cooling system is used.
Previously, an engine was devised and patented that has a tandem-piston arrangement in which the mode of operation is to induct a fresh fuel-air charge into a cavity that is opened between the lower surface of a motor piston and the upper surface of a charger piston as they descend in a cylinder. When the pistons rise, the fresh charge is compressed between the pistons in a space called the charger cavity for injection into the combustion chamber, which is located above the motor piston. Injection is through transfer channels running upward past the motor piston along the cylinder wall that are opened as the motor piston rises past them when it nears its zenith. Injection and combustion take place as the motor piston passes its top center position. The transfer channel openings are then closed by the lower motor-piston ring as it descends early in the power stroke while the upper surface of the charger piston is in tight full-surface contact with the lower surface of the motor piston. The patent number is U.S. Pat. No. 5,509,382; it issued in 1996, and it is titled: Tandem-Differential-Piston Cursive Constant-Volume Internal-Combustion Engine. For simplicity, the engine that is described in U.S. Pat. No. 5,509,382 will now be referred to as a tandem-piston engine.